Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. A number of metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures. X-ray scatterometry techniques offer the potential for high throughput without the risk of sample destruction.
Traditionally, optical scatterometry critical dimension (SCR) measurements are performed on targets consisting of thin films and/or repeated periodic structures. During device fabrication, these films and periodic structures typically represent the actual device geometry and material structure or an intermediate design. As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers. Optical metrology tools utilizing infrared to visible light can penetrate many layers of translucent materials, but longer wavelengths that provide good depth of penetration do not provide sufficient sensitivity to small anomalies. In addition, the increasing number of parameters required to characterize complex structures (e.g., FinFETs), leads to increasing parameter correlation. As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements.
In one example, longer wavelengths (e.g. near infrared) have been employed in an attempt to overcome penetration issues for 3D FLASH devices that utilize polysilicon as one of the alternating materials in the stack. However, the mirror like structure of 3D FLASH intrinsically causes decreasing light intensity as the illumination propagates deeper into the film stack. This causes sensitivity loss and correlation issues at depth. In this scenario, optical SCD is only able to successfully extract a reduced set of metrology dimensions with high sensitivity and low correlation.
In another example, opaque, high-k materials are increasingly employed in modern semiconductor structures. Optical radiation is often unable to penetrate layers constructed of these materials. As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.
In response to these challenges, more complex optical metrology tools have been developed. For example, tools with multiple angles of illumination, shorter illumination wavelengths, broader ranges of illumination wavelengths, and more complete information acquisition from reflected signals (e.g., measuring multiple Mueller matrix elements in addition to the more conventional reflectivity or ellipsometric signals) have been developed. However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).
Optical methods may provide non-destructive tracking of process variable between process steps, but regular calibration by destructive methods is required to maintain accuracy in the face of process drift, which optical methods cannot independently distinguish.
Atomic force microscopes (AFM) and scanning-tunneling microscopes (STM) are able to achieve atomic resolution, but they can only probe the surface of the specimen. In addition, AFM and STM microscopes require long scanning times. Scanning electron microscopes (SEM) achieve intermediate resolution levels, but are unable to penetrate structures to sufficient depth. Thus, high-aspect ratio holes are not characterized well. In addition, the required charging of the specimen has an adverse effect on imaging performance. X-ray reflectometers also suffer from penetration issues that limit their effectiveness when measuring high aspect ratio structures.
To overcome penetration depth issues, traditional imaging techniques such as TEM, SEM etc., are employed with destructive sample preparation techniques such as focused ion beam (FIB) machining, ion milling, blanket or selective etching, etc. For example, transmission electron microscopes (TEM) achieve high resolution levels and are able to probe arbitrary depths, but TEM requires destructive sectioning of the specimen. Several iterations of material removal and measurement generally provide the information required to measure the critical metrology parameters throughout a three dimensional structure. But, these techniques require sample destruction and lengthy process times. The complexity and time to complete these types of measurements introduces large inaccuracies due to drift of etching and metrology steps because the measurement results become available long after the process has been completed on the wafer under measurement. Thus, the measurement results are subject to biases from further processing and delayed feedback. In addition, these techniques require numerous iterations which introduce registration errors. In summary, device yield is negatively impacted by long and destructive sample preparation required for SEM and TEM techniques.
In semiconductor device manufacturing, etch processes and deposition processes are critical steps to define a device pattern profile and layout on a semiconductor wafer. Thus, it is important to measure films and patterned structures to ensure the fidelity of the measured structures and their uniformity across the wafer. Furthermore, it is important to provide measurement results quickly to control the on-going process and to adjust settings to maintain required pattern or film uniformity across the wafer.
In most examples, precise monitoring of a semiconductor manufacturing process is performed by one or more stand-alone (SA) metrology systems. SA metrology systems usually provide the highest measurement performance. However, the wafer must be removed from the process tool for measurement. For processes undertaken in vacuum, this causes significant delay. As a result, SA metrology systems cannot provide fast measurement feedback to process tools, particularly process tools involving vacuum. In other examples, integrated metrology systems or sensors are often attached to process equipment to measure wafers after a process step is completed, but without removing the wafer from the process tool. In other examples, in-situ (IS) metrology systems or sensors are employed inside a processing chamber of a process tool. Furthermore, an IS metrology system monitors the wafer during the process (e.g., etch process, deposition process, etc.) and provides feedback to the process tool performing the fabrication step under measurement.
In one example, structures subject to a reactive ion etch process are monitored in-situ. In some fabrication steps, the etch process is required to etch completely through an exposed layer and then terminate before substantial etching of a lower layer occurs. Typically, these process steps are controlled by monitoring the spectral signature of the plasma present in the chamber using an emission spectroscopy technique. When the exposed layer is etched through and the etch process begins to react with a lower layer, a distinct change in the spectral signature of the plasma occurs. The change in spectral signature is measured by the emission spectroscopy technique, and the etch process is halted based on the measured change is spectral signature.
In other fabrication steps, the etch process is required to etch partially through an exposed layer to a specified etch depth, and terminate before etching completely through the exposed layer. This type of etch process is commonly referred to as a “blind etch”. Currently, the measurement of etch depth through partially etched layers is based on near-normal incidence spectral reflectometry.
Current in-situ sensors are only capable of monitoring bulk changes to film thicknesses and do not correlate well to the complex profiles that result from the processing of deep 3-D structures.
In general, there are many methods of process monitoring using combinations of optical, acoustic and electron beam tools. These techniques measure the device directly, specially designed targets, or specific monitor wafers. However, the inability to measured parameters of interest of high aspect ratio structures in a cost effective and timely manner results in low yield, particularly in the memory sector of a wafer.
In summary, ongoing reductions in feature size and increased depth of many semiconductor structures imposes difficult requirements on metrology systems, including stand-alone systems and those integrated with process tools, such as ion implant and etch tools. Thus, improved metrology systems and methods are desired to measure high aspect ratio structures to maintain high device yield.